Cytochrome P450 3A4/5 enzymes (CYP3A4/5) collectively metabolize more drugs (licit and illicit) than any other cytochrome P450 enzymes (e.g. HIV protease inhibitors, methadone, cocaine). ? Our laboratory has found that CYPSA activity is elevated during late pregnancy in both women and mice (including in transgenic mice expressing the CYP3A4-promoter luciferase construct, Tg mice). While these data provide guidance on dose adjustment of CYPSA metabolized drugs during the Srd trimester, due to ethical and logistical reasons, no such data are available to guide dosing of CYPSA metabolized drugs during the 1st and 2nd trimesters. Therefore, our specific aims are to determine: la) if human hepatocytes exposed to Srd trimester plasma hormone concentrations can quantitatively replicate the in vivo induction of CYPSA (and other CYP) enzymes during the 3rd trimester: and lb) whether human hepatocytes can be used to predict the magnitude of change in in vivo hepatic CYPSA (and other CYP) activity in the 1st and 2nd trimester. To determine the mechanisms by which pregnancy induces CYPS/VCypSa activity, we will pursue the following specific aims: 2) We will confirm that placental growth hormone (or mouse growth hormone), progesterone, Cortisol (corticosterone) or estradiol, at their physiological concentrations observed during late mouse or human pregnancy, produce comparable induction of hepatic luciferase, CypSa activity and CypSa mRNA in hepatocytes from female Tg mice of reproductive age; S) Using the combination of hormones identified in Aims la and 2 above, we will determine its ability, after chronic administration, to induce hepatic luciferase, CypSa activity and CypSa mRNA expression, in vivo, in non-pregnant female Tg mice (i.e. pseudopregnancy studies); 4) To determine if the hormones in Aim S induce CypS/VSa enzymes via their receptors, we will determine their ability to induce CypSA/Sa activity and expression in: a) Human hepatocytes where their receptors (e.g. GR) have been knocked-down by siRNA technology; b) The most appropriate hepatic specific null receptor pregnant mice (e.g. GR(-/-) or GHR/GR(-/-)) as determined by results obtained from Aim 4a.